Angioplasty stents

ABSTRACT

An angioplasty stent comprises a body comprising a plurality of successive segments connected in pairs by bridge means so that the successive segments can be oriented relative to one another for the purposes of bending of the body in any direction defined by a linear combination of respective orientation axes defined by the bridge connection means. During the radial expansion of the stent, the axial contraction of the segments resulting from the opening-out of the respective loops is compensated by axial projection of the bridge elements from the respective concave portions. The wall of the body comprises arms for supporting a lumen as well as regions which are selectively deformable during the expansion of the stent, the arms and the selectively deformable regions having different cross-sections and/or cross-sectional areas. At least one portion of the body may have a substantially reticular structure, the branches of which define geometrical figures identifiable as fractals.

This application is a continuation of application Ser. No. 12/559,016,filed Sep. 14, 2009, which is a continuation of application Ser. No.11/784,912, filed Apr. 10, 2007, now abandoned, which is a continuationof application Ser. No. 11/136,002, filed May 24, 2005, now U.S. Pat.No. 7,267,684 B2, issued Sep. 11, 2007, which is a continuation ofapplication Ser. No. 10/626,292, filed Jul. 24, 2003, now U.S. Pat. No.6,896,698 B2, issued May 24, 2005, which is a continuation ofapplication Ser. No. 10/002,783, filed Oct. 30, 2001, now U.S. Pat. No.6,616,690 B2, issued Sep. 9, 2003, which is a continuation ofapplication Ser. No. 08/964,158, filed Nov. 4, 1997, now U.S. Pat. No.6,309,414, issued Oct. 30, 2001, the contents of each of which arehereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates in general to so-called stents forangioplasty.

BACKGROUND OF THE INVENTION

The term “stent” is intended to indicate in general a device to befitted in a lumen (for example, inside a blood vessel), usually bycatheterization, and subsequently spread out in situ in order to supportthe lumen locally. This has the main purpose of preventing there-establishment of a stenotic site in the location treated. It should,however, be pointed out that it has already been proposed in the art touse substantially similar structures for spreading-out and anchoringvascular grafts in situ; naturally this possible extension of the fieldof application is also intended to be included in the scope of theinvention.

For a general teaching with regard to vascular stents, reference mayusefully be made to the work “Textbook of Interventional Cardiology” byEric J. Topol, W.B. Saunders Company, 1994 and, in particular, toSection IV of Vol. II, entitled “Coronary stenting”.

A large number of patent documents are also dedicated to the subject asis shown, for example, by U.S. Pat. No. 4,776,337, U.S. Pat. No.4,800,882, U.S. Pat. No. 4,907,336, U.S. Pat. No. 4,886,062, U.S. Pat.No. 4,830,003, U.S. Pat. No. 4,856,516, U.S. Pat. No. 4,768,507, andU.S. Pat. No. 4,503,569.

In spite of extensive research and experimentation as documented at thepatent level, only a very small number of operative solutions has up tonow been used in practice.

This fact can be attributed to various factors, amongst which thefollowing problems or requirements may be mentioned:

-   -   to ensure that, during its advance towards the site to be        treated, the stent can adapt in a sufficiently flexible manner        to the path along which it is traveling even with regard to        portions having small radii of curvature such as those which may        be encountered, for example, in some peripheral vessels; this        must be achieved without adversely affecting the ability of the        stent to perform an effective supporting action once positioned        and spread out,    -   to prevent, or at least limit the effect of longitudinal        shortening which occurs in many stents when they are spread out,    -   to offer as broad as possible a bearing surface to the wall of        the lumen to be supported,    -   to avoid giving rise to complex geometry and/or to possible        stagnation sites which, particularly in applications in blood        vessels, may give rise to adverse phenomena such as coagulation,        clotting, etc., and    -   to reconcile the requirements set out above with simple and        reliable production methods and criteria, within the scope of        currently available technology.

SUMMARY OF THE INVENTION

The object of the present invention, which has the specificcharacteristics claimed in the following claims, is to solve at leastsome of the problems outlined above.

In one aspect, this invention is an angioplasty stent comprising a bodywhich has a generally tubular envelope and can be expanded in use from aradially contracted condition towards a radially expanded condition,said body comprising a plurality of successive segments connected inpairs by bridge means, each of the bridge means defining a connectingrelationship between two of the segments with a capability for relativeorientation identified by at least one respective orientation axis, sothat the successive segments can be oriented relative to one another forthe purposes of bending of the body in any direction defined by a linearcombination of respective orientation axes defined by the bridgeconnection means.

In another aspect, this invention is an angioplasty stent comprising abody which has a generally tubular envelope and can be expanded in usefrom a radially contracted condition towards a radially expandedcondition, wherein:

-   -   the body comprises a plurality of generally annular segments,        the wall of each segment being defined by a plurality of loops,        and    -   at least some of the segments are interconnected by bridge        elements extending in the general direction of the longitudinal        axis of the stent and having at least one end connected to the        concave or inside portion of a respective loop so that, during        the radial expansion of the stent, the axial contraction of the        segments resulting from the opening-out of the respective loops        is compensated by axial projection of the bridge elements from        the respective concave portions.

In another aspect, this invention is an angioplasty stent comprising abody which has a generally tubular envelope and can be expanded in usefrom a radially contracted position towards a radially expandedcondition in which the stent supports the wall of a lumen, wherein thewall of the body comprises arms for supporting the lumen, as well asregions which are selectively deformable during the expansion of thestent, and in that the arms and the selectively deformable regions havedifferent cross-sections and/or cross-sectional areas.

In yet another aspect, this invention is an angioplasty stent comprisinga body which has a generally tubular envelope and can be expanded in usefrom a radially contracted condition towards a radially expandedcondition, wherein:

-   -   the body comprises a plurality of successive radially expandable        segments interconnected by bridge elements extending        substantially in the direction of the longitudinal axis of the        stent so that the bridge elements are substantially unaffected        by the radial expansion of the segments and the bridge elements        are generally deformable in the direction of the longitudinal        axis so that the length of the stent along the axis can change        substantially independently of the radial expansion.

And in yet another aspect, this invention is an angioplasty stentcomprising a body which has a generally tubular envelope and can beexpanded in use from a radially contracted condition towards a radiallyexpanded condition, wherein at least one portion of the body has asubstantially reticular structure, the branches of which definegeometrical figures identifiable as fractals.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, purely by way of non-limitingexample, with reference to the appended drawings, in which:

FIG. 1 is a general perspective view of a first angioplasty stent formedin accordance with the invention,

FIG. 2 is a side view of the stent of FIG. 1, on a slightly enlargedscale,

FIG. 3 shows the geometrical characteristics of the wall of the stent ofFIGS. 1 and 2 in an imaginary development in a plane,

FIG. 4, which is generally comparable to FIG. 3, shows a first variantof the stent generally similar to that shown in FIGS. 1 and 2,

FIGS. 5 and 6 are two sections taken on the lines V-V and VI-VI of FIG.4, respectively,

FIG. 7 is a perspective view of another angioplasty stent,

FIG. 8 is a side view of the stent of FIG. 7,

FIG. 9 shows essentially in the same manner as FIG. 3, an imaginarydevelopment in a plane of the wall of the stent of FIGS. 7 and 8, and

FIGS. 10 and 11 show further possible developments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although several variants are referred to, the reference numeral 1 isused for generally indicating a so-called angioplasty stent FIGS. 1, 2,7 and 8.

For a general identification of the method of use and the structuralcharacteristics of an implant of this type, reference should be made tothe documentation cited in the introductory part of the description.

In summary, it will be remembered that the stent 1 is usually producedin the form of a body with a tubular envelope having an overall lengthof between a few millimeters and a few tenths of a millimeter, a wallthickness (the wall usually having a mesh or loop structure withopenings, as will be explained further below) of the order, for example,of a few hundredths of a millimeter, in view of its possible insertionin a lumen (such as a blood vessel) in a site in which a stenosis is tobe remedied. The stent is normally put in position by catheterization,after which radial expansion from an insertion diameter of the order,for example of 1.5-1.8 mm to an expanded diameter, for example, of theorder of 3-4 mm takes place in a manner such that, in the expandedcondition, the stent supports the lumen, preventing the recurrence of astenosis. In general, the outside diameter in the radially contractedcondition is selected so as to allow the stent to be introduced into thelumen being treated, whereas the expanded diameter corresponds ingeneral to the diameter to be maintained and established in the lumenonce the stenosis has been eliminated. It should again be pointed outthat, although the main application of the stents described relates tothe treatment of blood vessels, its use as an element for supporting anylumen in a human or animal body can certainly be envisaged (and istherefore included within the scope of the invention).

With regard to the methods and criteria which enable the stent to bespread out (that is, expanded in situ), the solution which is currentlymost widespread is that of the use of a so-called balloon catheter, thestent being disposed around the balloon of the catheter in thecontracted condition and the balloon then being expanded once the stenthas been brought to the site in which it is to be positioned. However,other solutions are possible, for example, the use of superelasticmaterials which cause the stent to expand once the restraining elements,which are intended to keep the stent in the contracted condition untilthe implant site has been reached, are removed. In addition oralternatively, the use of materials having so-called “shape memory” toform the stent so as to achieve the radial expansion in the implantposition has also been proposed.

Usually (for more precise indications, reference should be made to thebibliographical and patent documentation cited in the introduction tothe description) the stent is made of metal which can reconcile twobasic requirements for the application, that is, plastic deformabilityduring the expansion stage and the ability to withstand any stresseswhich would tend to cause the stent to close up, preserving the expandedshape. The material known by the trade name of “Nitinol” is well knownand also has super-elasticity and shape-memory properties which may berequired in the expansion stage.

In any case, these technological aspects will not be dealt with indetail in the present description since they are not relevant per se forthe purposes of understanding and implementing the invention. This alsoapplies essentially to the technology for the production of the stentsaccording to the invention. As already stated, in general terms, theseadopt the appearance of bodies with tubular envelopes having walls withopenings. With regard to the production methods, according to the priorart, at least three basic solutions may be used, that is:

-   -   forming the stent from a continuous tubular blank to be cut up        into individual stents, the walls with openings being formed by        techniques such as laser cutting, photo-engraving,        electron-discharge, machining, etc;    -   producing the stent from a strip-like body in which the regions        with openings are formed, for example, by the techniques        mentioned above, with a view to the subsequent closure of the        strip-like element to form a tube, and    -   producing the stent from a metal wire shaped by the successive        connection of loops of wire, for example, by means of        micro-welding, brazing, gluing, crimping operations, etc.

The first solution described is that which is currently preferred by theApplicant for producing stents according to the embodiments describedbelow, with the exception of the solution to which FIG. 4 relates whichintrinsically involves the use of a metal wire. In particular,laser-beam cutting has been found the most flexible solution with regardto the ability to modify the characteristics of the stents quicklyduring production according to specific requirements of use.

In any case, it is stressed, that this production aspect is of onlymarginal importance for the purposes of the implementation of theinvention in the terms which will be recited further below, particularlywith reference to FIG. 4. This also applies with regard to the selectionof the individual techniques and of the order in which the various stepsdescribed (the production of the walls with openings, parting, anybending of the strip-like element, etc.) are carried out.

In all of the embodiments described herein, the body of the stent 1extends in a longitudinal direction generally identified by an axis z.For clarity, it should however be pointed out that the stent is intendedto be bent, possibly significantly, during use, easy flexibilityactually being one of the characteristics sought.

In all of the embodiments described herein, the body of the stent 1comprises a series of successive, generally annular segments, indicatedas 2 in the drawings. As can easily be seen, the stent 1 of FIGS. 1 and2 comprises seven of these segments, whereas the stent of FIGS. 7 and 8comprises six.

By way of indication, although this should not be interpreted aslimiting of the scope of the invention, the length of the segments 2measured longitudinally of the stent 1, and hence along the axis z, isof the order of about 2 mm. In other words, for reasons which willbecome clearer from the following, the segments 2 are quite “short”lengthwise.

As can be appreciated best in the side view of FIG. 2, the varioussegments of the stent 1 shown therein are connected to one another bypairs of bridges 3, 4 (actually constituting integral parts of the stentwall, as will be explained further below, for example, with reference toFIG. 3) the essential characteristic of which (this applies both to thestent of FIGS. 1 and 2 and to the stent of FIGS. 7 and 8) is toarticulate the segments 2 connected respectively thereby in analternating sequence about mutually perpendicular flexing or bendingaxes.

This type of solution achieves two advantages.

In the first place, the longitudinal flexibility of the stent 1 which isnecessary to facilitate its location at the implantation site, isdemanded essentially of the bridges 3, 4, whereas the structuralstrength and hence the support for the lumen is demanded of the actualstructures of the segments 2; all of this is achieved with a capabilityto optimize the desired characteristics by precise adaptation of thesections of the various component elements.

In the second place, the arrangement of the bridges in a sequence(usually, but not necessarily, alternating), in combination with thefact that, as stated, the segments 2 are quite short, enables a bend tobe formed easily, in practice, at any point along the length of thestent 1 in any direction in space, and with very small radii ofcurvature.

This concept can be understood more easily with reference to thesolution of FIGS. 1 and 2 (as will be explained, the same also appliesto the solution according to FIGS. 7 and 8) if it is noted that, byvirtue of their arrangement at 180° in diametrically opposed positionson the wall of the stent 1, the bridges 3 allow the stent 1 to bendlocally about a respective axis x generally transverse the axis z.

The bridges 4, which are also arranged at 180° to one another in a planeperpendicular to that of the bridges 3, allow the stent 1 to bendlocally about a second axis y transverse the longitudinal axis z and, inthe embodiment shown, perpendicular to the above-mentioned axis x.

Since, as already stated, the segments 2 are quite short, the aforesaidaxes x and y are arranged in close proximity to one another inalternating sequence along the length of the stent 1, however manysegments 2 there may be.

As a result, the stent can easily be bent, in practically anylongitudinal position of the stent 1, about a generic axis d which canbe defined on the basis of an equation such as

{right arrow over (d)}={right arrow over (ax)}+{right arrow over(by)}  (1)

that is, as a linear combination of the bending movements about the axesidentified by the vectors

{right arrow over (x)} and {right arrow over (y)}.

With reference to the general theory of vectorial spaces, it can alsoeasily be understood that the availability of respective capabilitiesfor bending along two perpendicular axes in sequence, preferably inalternating sequence, constitutes the simplest solution for achievingthe desired object. Solutions in which successive segments 2 of thestent 1 are connected by bridges such as the bridges 3 and 4 (or byelements which provide for similar bending capabilities, as will beexplained further below with reference to FIGS. 7 and 8) in the regionof axes which are not mutually perpendicular would however, at least inprinciple, be possible. A solution in which, for example, pairs ofdiametrally-opposed bridges arranged in sequence and spaced apartangularly by 60° may be mentioned by way of example.

Moreover, the alternating sequence described above, that is: axis x,axis y, axis x, axis y may, at least in principle, be replaced by adifferent sequence, for example, axis x, axis x, axis y, axis y, axis x,axis x, etc. Provision for a capability to bend about the axis x in twoadjacent segments 2 followed by a capability to bend about the axis yrepeated for two adjacent segments 2, as in the latter example mentionedmay, in fact, be advantageous in applications in which an ability toachieve very small radii of curvature is to be given preference.

In the solution of FIGS. 7 and 8, the same conceptual solution isachieved in a slightly different manner.

In the solution shown in FIGS. 7 and 8, the various segments 2 are infact connected to one another by means of bridges forming respectiveportions of two “spines” of the stent constituted by integral parts ofthe stent 1 which extend along a generally winding path along twogeneratrices of the imaginary cylindrical surface of the stent indiametrally opposed positions. The respective structural details willbecome clearer from the description given below.

From an observation, in particular, of FIG. 8 and with the use of thesame conventions as were used with reference to FIG. 2, it can be seenthat the flexibility in the region of respective loops extending betweensuccessive segments 2 provided for by the spines 30 achieves the localflexibility about the axis y relative to the general direction definedby the axis z.

The local extensibility of the aforementioned bridges and, inparticular, the ability of one of the bridges to extend while thediametrally-opposed bridge retains approximately correspondinglongitudinal dimensions, or extends to a more limited extent, orpossibly contracts slightly longitudinally, enables the bending movementabout the axis x to be achieved, as indicated schematically by a brokenline for the segment 2 which is farthest to the left in FIG. 8.

In this embodiment, the stent 2 can thus also be bent in the location ofeach connection between adjacent segments 2 about a generic axis ddefined by an equation such as equation (I) introduced above.

As will be appreciated once again, all of this is achieved while thestructure of the individual segments 2 remains substantially unchangedand thus in a manner such that the longitudinal bending of the stent 1can be attributed essentially to the bending and/or, in general, to thelocal deformation, solely of the bridges connecting adjacent segments 2.

With reference to FIG. 3, it can be seen that, as already indicatedabove, this constitutes an imaginary development in a plane, reproducedon an enlarged scale, of the wall of the stent of FIGS. 1 and 2.

In fact, the seven segments 2 connected in alternating sequence by thepairs of bridges 3 and 4 arranged in pairs of diametrally-opposedelements disposed at 90° in alternating sequence can be seen in FIG. 3.As already stated, this is an imaginary development in a plane which maycorrespond to the development of a strip-like blank from which the stent1 is then produced by bending of the blank to form a tube.

It can also be noted from an observation of FIG. 3 that the generallyannular body of each segment 2 comprises in the embodiments shown, a setof approximately sinusoidal loops of substantially uniform size(measured circumferentially relative to the element 2) which is doubledin the region of the loops from which the bridges 3, 4 extend, in themanner explained further below.

It is possible to recognize, within each segment 2, a respectiveimaginary median plane X2 which, in the embodiments illustrated, isgenerally perpendicular to the longitudinal axis z. Two of these planes,indicated X2 are shown in FIG. 3 (and in FIG. 9); naturally, since theseare developments in a plane, the median planes in question arerepresented in the drawings by straight lines.

It can thus be noted that each segment 2 comprises a sequence of loops,each loop (approximately comparable to half of a sinusoidal wave)defining a respective concave portion 5, the concave side of which facestowards the median plane X2, and which is connected to two approximatelystraight arms 6.

By way of indication, only two of these loops interconnected by a bridge3 have been marked specifically in FIG. 3. In particular, these are thetwo loops of which the concave portions are indicated 5 and the lateralarms are indicated 6.

It can easily be understood that the radial expansion of the stent 1takes place substantially as a result of an opening-out of theaforementioned loops; by way of indication, with reference to thedevelopment in a plane of FIG. 3, the radial expansion of the stentcorresponds to a stretching of the development in a plane shown in FIG.3 in the sense of an increase in height and hence a vertical expansionof FIG. 3.

In practice, this radial expansion corresponds to an opening-out of theconcave portions 5, whereas the lateral arms 6 of each loop remainsubstantially straight.

The localization of the plastic deformation of the stent 2 in theconcave portions of the loops 5 may be favored (as will be explainedfurther below with reference to FIG. 4) by means of the cross-sectionsand/or the cross-sectional areas of the portions of each loop.

In any case, the radial expansion (vertical stretching of thedevelopment in a plane of FIG. 3) affects essentially the concaveportions 5 of the loops of the elements 2 and in no way affects thebridges 3, 4 which extend longitudinally (axis z).

It will be appreciated that the same also applies to the solution shownin FIG. 4 (which will be referred to further below) in which one of themedian planes X2 has been shown, only one of the loops being indicatedand its concave portion 5 and its lateral arms 6 being identifiedspecifically. The same criterion also applies to the solution of FIGS. 7and 8; in this connection, reference should be made to the developmentin a plane of FIG. 9. In this drawing, as in FIG. 3, two median planesX2 of two segments 2 have been indicated, and the concave portion 5 andthe lateral arms 6 of two opposed loops, between which a portion of oneof the sinusoidal spines 30 extends like a bridge, are also shown.

As can be seen best from a comparison of FIGS. 3, 4 and 9, a featurecommon to all of the solutions described is that the radial expansion ofthe segments 2 corresponds, within each segment 2, to an imaginarymovement of the concave portion 5 of each loop towards the median planeX2 of the segment 2 of which this loop forms part.

Anyone reading this description can easily perceive this, for example,by thinking of the segment 2 corresponding to the plane X2 farthest tothe right in FIG. 3 as extending vertically. As a result of thisstretching, carried out precisely along the line X2 which identifies theaforesaid plane, the concave portion 5 of the loop indicated in factmoves towards the line X2, the same behavior being followed, in oppositedirections, according to their different locations relative to the lineX2, by the concave portions of all of the other loops.

If, with reference to the bridges 3 (and the same also applies to thebridges 4 as well as to the individual portions of the spines 30 whichdefine the parts equivalent to the bridges 3 and 4 in FIG. 9), it isconsidered that the connection to the relative segments 2 is formed inthe region of the concave (or inside) portion of a respective loop, itcan easily be appreciated that the radial expansion of the segments 2 isaccompanied, so to speak, by a thrust exerted on the bridges 3, 4 (andon the respective spine portions 30). This thrust corresponds, so tospeak, to an expulsion of the bridges or of the spine portions inquestion from the corresponding segment 2.

To concentrate attention once again on the segment 2 the median plane X2of which is farthest to the right in FIG. 3, if the segment 2 inquestion is thought of as being stretched vertically, it will be seenthat, as a result of the movement of the concave portions (such as, forexample, the concave portion indicated 5 in the segment 2 in question)towards the plane X2, the respective bridges 3 tend to move towards theleft relative to the median plane X2 of the corresponding segment 2.

This expulsion effect on the bridges 3 is beneficial for eliminating thetendency demonstrated by many stents of the prior art to contractlongitudinally during radial expansion.

By the adoption of a geometry such as that shown in FIGS. 3 and 4, forexample, the axial contraction of the segments 2 resulting from theirradial expansion is in fact compensated (and possibly even overcome) bythe above-described “expulsion” of the bridges 3 and 4. Tests carriedout by the Applicant show, in this connection, that, a geometry, forexample, such as that illustrated in FIGS. 3 and 4 causes the stent 1not only not to shorten but, on the contrary, to lengthen slightlyduring the radial expansion.

The explanation of this mechanism is quite simple. In this connection,it suffices to consider, again with reference to FIG. 3, what wouldhappen if, theoretically, instead of being located where they are shown(and thus connecting respective concave loop portions of adjacentsegments 2), the bridges 4 of two aligned loops of two adjacent segments2 were arranged as indicated by broken lines and indicated 4′ and hencenot connecting concave (inside) loop portions but connecting convex (oroutside) loop portions.

The bridges 4′ indicated above are extremely short (it will beremembered, by way of reference, that the axial length of the segments 2may be of the order of 2 mm). Even during radial expansion, the concaveportions (and consequently the convex portions) of all of the loops ofeach segment in any case retain their alignment with a plane parallel tothe median plane X2 at each end of each segment 2. This alignment isthus also retained by the concave or convex portions connected betweentwo adjacent segments 2 by the same bridge 3, the length of which is notchanged during the radial expansion.

Consequently, the length of a stent in which the bridges 3 were arrangedas shown in FIG. 3 and the bridges 4 as schematically indicated 4′,again in FIG. 3, (naturally with reference to all of the pairs ofbridges 4 present in the stent) would remain practically unchangedduring radial expansion.

On the other hand, as already stated, with the use of the geometry shownin FIG. 3, owing to the superimposition of the various deformationmovements, the axial length of the stent 1 is not merely kept constantbut even increases slightly. It will also be understood from theexplanation given above that, even though the location of the bridgesindicated 4′ is mentioned theoretically for explanation, it couldactually be used, according to specific requirements. Moreover, it caneasily be understood from the foregoing explanation that theconservation of the axial length during the radial expansion does notnecessarily require all of the bridge elements (which are not affectedby the deformation resulting from the radial expansion) to be connectedto concave or inside portions of respective loops of the segments 2. Infact it suffices, for this purpose, for one such connection to beprovided for each longitudinal section which is intended to contractlongitudinally as a result of the radial expansion of the stent 1.

For example, in the embodiment shown by solid lines in FIG. 3, (the samealso applies to the embodiment of FIGS. 4 and 9) each of the segments 2comprises a section which is intended to contract longitudinally as aresult of the radial expansion. A corresponding connection of bridges 3,4 is therefore provided in each of these segments according to thecriteria described above.

With reference, on the other hand, to the connection arrangement of thebridges 4′ indicated primarily for didactic purposes in FIG. 3, it canbe noted that each set of two segments 2 interconnected by respectivebridges 3 constitutes, precisely for the reasons described above, asection of stent which does not contract substantially longitudinallyduring radial expansion. For this reason, the connection between thesesections can take place by means of bridges such as those which areindicated 4′ and shown in broken outline in the drawing and which arenot connected to a concave (or inside) loop portion.

It can be noted from an examination of the diagram of FIG. 9, that allof the segments 2 illustrated therein can contract axially as a resultof radial expansion. For this reason, the bridges defined by thesinusoidal spines 30 and connecting adjacent segments 2 satisfyprecisely the condition described above, that is, connection to theconcave portion of a respective loop.

With regard to the general geometry, the variant of FIG. 4 againproposes the connection arrangement described above with reference toFIG. 3.

FIG. 4 shows how a stent wall structure having the geometry describedwith reference to FIG. 3 can also be formed from one or more pieces ofwire bent so as to form a set of loops which is substantially similar tothat shown in FIG. 3, and in which the bridges 3 and 4 comprise wireportions which are coupled (that is, placed side by side parallel to oneanother) and connected, for example, by welding or other joining methods(for example, brazing, gluing, crimping, etc.).

The use of a wire enables different cross-sections and/orcross-sectional areas to be attributed (for example, by a mechanicaloperation to shape the wire) to the concave portions 5 of the loops andto the straight arms 6 which extend therefrom. For example, it canreadily be appreciated that the cross-section of FIG. 5 is in fact thecross section of a concave portion taken in its tip portion, whereas thecross-section of FIG. 6 corresponds to the connection region of twostraight arms extending generally longitudinally relative to the stent(axis z).

In particular, in the concave portions of the loops, the wireconstituting the stent wall may retain a round cross-section, but in thestraight portions 6 may adopt a cross-section which is generallyflattened in the plane of the wall (and hence along the imaginarycylindrical envelope) of the stent 1.

This different shaping enables various results to be achieved.

The straight portions 6 are intrinsically more resistant to bending inthe plane in which they are generally flattened so that a force openingout the two arms 6 connected to a common concave portion 5 brings abouta deformation of the loop in the concave portion 5. Although the arms 6are opened out, they retain a generally straight shape; in thisconnection, it will be noted that the arms which are coupled to form thebridges 3 and 4 nevertheless retain a straight orientation along thelongitudinal axis z of the stent 1.

By virtue of their flattened shape the arms 6 expose a wider surface tothe wall of the lumen supported by the stent in its radially expandedcondition. The wall of the lumen is therefore subjected to a distributedload preventing the formation of concentrated stress regions.

The dimensions of the wire can be optimized in the concave portions 5 inorder to achieve optimal characteristics of plastic deformability whenthe stent is expanded radially and, at the same time, resistance tosubsequent stresses which may tend to close up the stent 1.

It should in any case be pointed out, for clarity, that the solution ofmaking the cross-sections and/or the cross-sectional areas of thevarious parts of the stent wall different in the terms illustrated byway of example with reference to FIG. 4 is also practicable in thesolutions described with reference to FIGS. 3 and 9, (although withtechnological solutions other than the mechanical squashing of the wirementioned with reference to FIG. 4).

To examine this latter solution, and with further reference to theperspective and elevational views of FIGS. 7 and 8, it can be notedthat, as a general rule, the structure and shape of the loopsconstituting the various segments 2 is generally similar to thatdescribed above with reference to FIGS. 3 and 4. In particular, asindicated in the segment 2 situated farthest to the left, it is alsopossible generally to distinguish in the loops shown in FIG. 9 a concave(or inside) portion 5, extending from which are two straight arms 6which are intended to be opened out when the stent 1 is expandedradially.

The wall structure of FIG. 9 differs from that shown in FIGS. 3 and 4essentially in that the bridges which interconnect the various segments2 comprise the two spines 30 extending with a generally sinusoidal shapealong two diametrally opposed generatrices of the structure of the stent1.

Naturally, the presence of two of these spines does not constitute anessential choice. For example, instead of having two spines 30 which arediametrally opposed (and hence spaced apart angularly by 180°) it ispossible to use a single spine of this type or three spines spacedangularly by 120° etc.

In any case a structure with spines of the type described can alsoimplement an equation such as equation (I) given above, for the purposesof the longitudinal bending of the stent 1. The difference in comparisonwith the embodiment shown in FIGS. 1 to 4 lies in the fact that, in thisfirst solution, the axes x and y in fact correspond to the axes aboutwhich the bending of the pairs of bridges 3, 4 can take place. In theembodiment of FIGS. 7 to 9, on the other hand, (in this connection seealso the elevational view of FIG. 8), each section of the spine 30extending to connect two adjacent segments 2 can express twopossibilities for relative orientation between the two segments 2connected, that is:

-   -   twisting or, more correctly, bending in the general plane of the        spine 30, and    -   extension, or in general, variation in length in this plane.

This concept may become clearer to experts in mechanics if it is notedthat, in practice, both the solution illustrated in FIGS. 1 to 4 and thesolution illustrated in FIGS. 7 to 9 form structures generallycomparable to that of a universal joint.

The generally sinusoidal shape of the two spines 30 enables thelongitudinal extensibility of the spines to be utilized for bendingpurposes without giving rise to stresses which are oriented tangentiallyrelative to the wall of the stent and hence risk giving rise toundesired twisting. It will, in any case, be appreciated that the lengthof the stent of FIGS. 7 to 9 (that is, its extent along the axis z) canchange entirely independently of the radial expansion of the segments 2.This can easily be seen if it is noted that the overall shape of thespines 30 is sinusoidal and, even where they are connected to concaveportions of respective loops (see in particular the portion of the spine30 which is shown in the lower portion of FIG. 9 connecting the twoelements 2 of which the median planes X2 are indicated) the connectionwith these concave portions 5 does not change the general sinusoidalshape of the spine in question. In other words, the spine 30 isconnected to the outer edge of the outside of the concave portion 5 onone side or wall thereof and continues from the inside of the concaveportion, from the opposite side or wall.

The solution described provides for the entire body 1 of the stent, orat least part of it, to comprise a substantially reticular structure,the branches of which (in the embodiment shown, the annular walls of thesegments 2 and the two spines 30) define geometrical figures which canbe identified as fractals.

The term “fractal”, coined by the mathematician B. Mandelbrot in 1975,indicates in general a geometrical figure which has internal symmetriesto whatever scale it is enlarged, and which is produced as a limitconfiguration of a succession of fragmentary curves from each of whichthe next is obtained on the basis of an assigned rule, for example, byreplacing each side with a predetermined fragmentary, so-calledgenerative or generator line.

Solutions such as those shown by way of example in FIGS. 2, 7 and 9 canbe developed with the use of higher-order fractals, as shownschematically in FIGS. 10 and 11.

In particular, FIG. 10 shows, by way of example, the use of higher-orderfractals to produce segments 2, and FIG. 11 shows, by way of example,the use of higher-order fractals to produce the spine or spines 30.Clearly the solutions shown by way of example may be combined, in thesense that the higher-order fractals may be used both for the segments 2and for the spines 30.

In any case, the use of fractal geometry has been found advantageoussince it enables the performance and/or the mechanical characteristicsof the various portions of the wall of the stent 1 to be optimized withregard to the specific stresses to which it has to respond in use.

1. An angioplasty stent comprising a body which has a generally tubularwall and configured to be expandable in use from a radially contractedcondition towards a radially expanded condition, wherein the wall of thebody comprises a plurality of arms and a plurality of selectivelydeformable regions extending between the arms, each selectivelydeformable region having first and second ends connected between twoarms and being configured to be deformable during the expansion of thestent such that an angle between the two arms connected to eachselectively deformable region is greater in the radially expandedcondition than in the radially contracted condition, each arm having afirst cross-section, each deformable region having a secondcross-section which is different from the first cross-section, thecross-section of each selectively deformable region being substantiallyconstant except at transition portions adjacent the ends of theselectively deformable regions.
 2. The stent of claim 1, wherein eachselectively deformable region comprises a concave portion, and whereinthe concave portions and the arms differ in cross-sectional area sothat, during radial expansion of the stent, the concave portions and thearms adopt different behaviors.
 3. The stent of claim 2, wherein thearms have a shape which is generally flattened in the general plane ofthe wall so that outer faces of the arms define respective broad supportsurfaces for the wall of the lumen.
 4. The stent of claim 2, wherein theconcave portions have a substantially circular cross-section.
 5. Thestent of claim 1, wherein the wall of the body comprises at least onebent wire element.
 6. The stent of claim 5, wherein the wall comprisesadjacent portions of wire firmly connected to one another.
 7. The stentof claim 6, wherein adjacent portions of wire are connected to oneanother by one of welding, brazing, gluing, crimping or combinationsthereof.
 8. The stent of claim 1, wherein the arms are configured toextend substantially parallel to a longitudinal axis of the body whenthe body is in the radially contracted condition.
 9. The stent of claim8, wherein the arms have a first outer wall surface width and theselectively deformable regions have a second outer wall surface width,the first outer wall surface width being greater than the second outerwall surface width.
 10. The stent of claim 1, wherein each arm has afirst cross-sectional area and each deformable region has a secondcross-sectional area, the first cross-sectional area being differentfrom the second cross-sectional area.
 11. The stent of claim 2, whereineach concave portion has a concave side which faces a planeperpendicular to a longitudinal axis of the body.
 12. The stent of claim11, wherein each concave portion is configured to open outwardly whenthe body is expanded from the radially contracted condition to theradially expanded condition such that an angle between the two armsconnected to each selectively deformable region is greater in theradially expanded condition than in the radially contracted condition,each arm being substantially parallel to a longitudinal axis of the bodywhen the body is in the radially contracted condition.
 13. The stent ofclaim 2, wherein each concave portion is configured to open outwardlywhen the body is expanded from the radially contracted condition to theradially expanded condition, each of the concave portions having across-sectional area which is substantially constant over its lengthbetween the two arms and which is different from a cross-sectional areaof each of the arms.
 14. The stent of claim 1, wherein the plurality ofarms and the plurality of selectively deformable regions form aplurality of loops, the plurality of loops including a first portion ofloops connected to an adjacent loop by a bridge connector and a secondportion of loops free of connection to an adjacent loop.
 15. The stentof claim 1, wherein the first and second ends of each selectivelydeformable region are connected between ends of two of the plurality ofarms, each arm having first and second ends and first and second sides,a distance between the first and second ends being a length of the armand a distance between the first and second sides being a width of thearm, the plurality of arms being sized and configured such that when thebody is in the contracted condition the plurality of arms are orientedgenerally in a longitudinal direction of a longitudinal axis of the bodyand a maximum circumferential distance between adjacent arms is lessthan the width of the arms.
 16. An angioplasty stent comprising a bodywhich has a generally tubular wall defining a longitudinal axis, thebody being expandable in use from a radially contracted conditiontowards a radially expanded condition, the wall of the body having aplurality of arms and a plurality of selectively deformable regions,each selectively deformable region having first and second endsconnected between ends of two of the plurality of arms, each arm havingfirst and second ends and first and second sides, a distance between thefirst and second ends being a length of the arm and a distance betweenthe first and second sides being a width of the arm, each arm having across-sectional shape, each selectively deformable region having across-sectional shape which is different from the cross-sectional shapeof the arms, the plurality of arms being sized and configured such thatwhen the body is in the contracted condition a maximum circumferentialdistance between adjacent arms is less than the width of the arms. 17.The stent of claim 16, wherein each selectively deformable regioncomprises a concave portion, and wherein the concave portions and thearms differ in cross-sectional area so that, during radial expansion ofthe stent, the concave portions and the arms adopt different behaviors.18. The stent of claim 17, wherein the arms have a shape which isgenerally flattened in the general plane of the tubular wall of the bodyso that outer faces of the arms define respective broad support surfacesfor the wall of the lumen.
 19. The stent of claim 17, wherein theconcave portions have a substantially circular cross-section.
 20. Thestent of claim 16, wherein the wall of the body comprises at least onebent wire element.
 21. The stent of claim 20, wherein the wall comprisesadjacent portions of wire firmly connected to one another.
 22. The stentof claim 21, wherein adjacent portions of wire are connected to oneanother by one of welding, brazing, gluing, crimping or combinationsthereof.
 23. The stent of claim 16, wherein the arms are configured toextend substantially parallel to a longitudinal axis of the body whenthe body is in the radially contracted condition.
 24. The stent of claim16, wherein the arms have a first width and the selectively deformableregions have a second width, the first width being greater than thesecond width.
 25. The stent of claim 16, wherein each arm has a firstcross-sectional area and each deformable region has a secondcross-sectional area, the first cross-sectional area being differentfrom the second cross-sectional area.
 26. The stent of claim 17, whereineach concave portion has a concave side which faces a planeperpendicular to a longitudinal axis of the body.
 27. The stent of claim26, wherein each concave portion is configured to open outwardly whenthe body is expanded from the radially contracted condition to theradially expanded condition such that an angle between the two armsconnected to each selectively deformable region is greater in theradially expanded condition than in the radially contracted condition,each arm being substantially parallel to a longitudinal axis of the bodywhen the body is in the radially contracted condition.
 28. The stent ofclaim 17, wherein each concave portion is configured to open outwardlywhen the body is expanded from the radially contracted condition to theradially expanded condition such that an angle between the two armsconnected to each selectively deformable region is greater in theradially expanded condition than in the radially contracted condition,each of the concave portions having a cross-section which issubstantially constant over its length between the two arms and which isdifferent from a cross-section of each of the arms.
 29. The stent ofclaim 16, wherein the plurality of arms and the plurality of selectivelydeformable regions form a plurality of loops, the plurality of loopsincluding a first portion of loops connected to an adjacent loop by abridge connector and a second portion of loops free of connection to anadjacent loop.
 30. The stent of claim 16, wherein when the body is inthe contracted condition the plurality of arms are oriented generally ina longitudinal direction of the longitudinal axis of the body.